1. Field of the Invention
The present invention relates to a compressed gas supply unit having a plurality of scroll compressors and capable of maintaining stable operation even under frequently changing loads.
2. Description of the Related Art
If a compressed gas supply unit has only one compressor, the compressed gas supply unit may become unable to produce and supply compressed gas to a gas recipient, should a failure occur in this one compressor. Thus, compressed gas supply units having a plurality of compressors which are capable of controlling the number of compressors under operation in accordance with the load (demand for compressed gas) to supply compressed gas as required by the gas recipient are known. Such compressed gas supply units achieve energy savings by controlling and minimizing the number of compressors under operation.
Japanese Patent Application Laid-open No. 2010-190197 discloses a compressed gas supply unit including a common reservoir tank for reserving gas discharged from a plurality of compressors, and a pressure sensor for detecting the pressure in the reservoir tank, wherein the number of compressors under operation is controlled in accordance with changes in the amount of consumed gas, which are determined from measurements by the pressure sensor of the pressure in the reservoir tank.
Conventional compressed gas supply units having a plurality of compressors can supply compressed gas to the gas recipient at a level required by the gas recipient by maintaining the pressure in the merge pipe on the discharge side of the compressors or the reservoir tank in accordance with load change (demand for compressed gas) at the required level. Thus, inverter devices that allow variation of compressor speed are provided. The inverter devices modulate the speeds of the compressors and adjust the pressure on the discharge side of the compressors to the level required by the gas recipient. One problem here is power consumption which increases because of poor efficiency caused by low speed operation, which is necessary to adapt to low load of the compressed air supply units.
Japanese Patent Application Laid-open No. H11-343986 discloses a compressed air production apparatus, having a plurality of motor-driven screw compressors arranged in parallel rows, and the respective discharge air systems merged into one pipe or air tank. This compressed air production apparatus includes an inverter device that allows for variable speed modulation of the motors for the respective screw compressors, a pressure sensor provided in the merge pipe or air tank, and a controller that controls the plurality of screw compressors to keep the pressure detected by the pressure sensor within a range of predetermined upper and lower pressure limits. The controller controls to operate only one of the plurality of screw compressors at varying speeds by means of the inverter device, while other screw compressors are either operated under full load conditions, or stopped. Thereby this compressed air production apparatus reduces power consumption, as making it substantially linearly proportional to the amount of air being used.
The compressed air production apparatus disclosed in Japanese Patent Application Laid-open No. H11-343986 enables a reduction in power consumption as compared to simple speed modulation of a plurality of compressors. However, under frequently changing loads, the apparatus is not capable of modulating the speeds of the respective compressors immediately in response to a load change to stably maintain the discharge pressure of the compressors at a level required by the gas recipient.
Scroll compressors offer merits of small size, light weight, low vibration and noise, lower in component count, and ease of production, and have found wide applications as small compressors in vehicle air conditioning compressor,
superchargers, and the like. The scroll compressors can promptly react to frequent load changes by being used in combination with inverter devices, as their speed-torque characteristics are suited to variable speed drive by inverter devices.